Since Stronghold is a little different than your standard Doom-mod, here is a little overview of how things work.

1.The training mission

When you start a new game, you will begin your journey at the training ground of the UAC base on Mars.
This mission is supposed to give you a little preview of how the gameplay is set up in this mod and also provides the
introduction/explanation for the Deadline game-mode!
Completing this mission will give you no benefits (other than learning how things work) and it can be ended at any time by "using" the door to the left of the starting point.
This way players won't have to go through the training mission again, if they already know what it is about.
Once the training course is completed (or cancelled) you will be sent to the main UAC installation on Mars; the Intermap!

2.The intermission map

This is the main HUB of the game where you will be able to choose your missions, buy equipment and talk to NPC's.
There is a lot to discover here, so take your time!

2.1.The shops

There are 4 main shops in the Intermap accessible right from the start:
-The Armory (Caco plate)
Here you'll get different kinds of (you guessed it) armor

-The enhancement shop (UAC cybernetics Inc.)
Here you can increase your maximum health and armor. Note that the enhancements will be gone as soon as you loose a life though!
There is a also the Healthpad 2000, wich heals you for free if you return from a mission barely alive.

Later in the game you will get access to more shops where you can buy even the most powerful (and expensive) items.
You have to unlock those by beating a certain Tier though!

2.2.NPC's

There are a lot of characters running around the base, some to wich you can talk to, indicated by a exclamation mark above their heads. To speak to a NPC, just step close to him/her and press "use". Sometimes you're given choices how to react to something they said, wich may influence the outcome of the conversation!
Talking to them isn't necessary to beat the game, but many have useful information for you, can provide a little background story or are simply amusing. So, speaking with the people around the base is definitely encouraged.

2.3.The missions

The various maps in Stronghold are seperated into 6 Tiers, each containing 5 missions except for the last one, wich has 8.
You have to complete all maps in one tier to gain access to its last mission (Usually introducing a new game-mode), wich will then open the next Tier for you provided you can complete the mission successfully!

Each level is represented through a door with a picture of the planet it takes place on above it. Walking towards that door will give you in-detail information about the map it belongs to, including:
- the name of the planet
- the name of the sector (STR02, STR03, etc.)
- the number of waves
- the game type (look here for info on the game types: http://forum.drdteam.org/viewtopic.php?f=124&t=4454)
- the reward you get when successfully beating the map
- the status of the mission (unfinished, finished, time attack beaten)
- the name of the mission, including a small description of the map

Each door also has 2 lights right and left of it. This also indicates the status of the map:
red means it is still unfinished
orange is for completed missions
green indicates that the time-attack mode has been beaten (more on that later)

To start a mission, simply walk into the Doorway!

3.Within a mission

3.1. Overview

Alright, now we get into the real meat of the game.
In case you don't know it yet, the main goal in Stronghold is to defend UAC outposts from hordes of enemys. Those monsters will attack in waves of various sizes and will try to reach their goal (depending on the game-mode), while still attacking any opposition (you and your comrades).
Once all monsters of a wave are dead, you have some time to restock your ammo, health, items and weapons before the next one starts.
Finish off every wave to sucessfully beat a mission!

Every killed enemy will also give you credits (depending on the monsters strength), as well as completing a map itself will give you a reward. Use that money wisely!

3.2. The life/prison system

Unlike Doom(2) and most modifications for it, Stronghold works with extralifes rather than expecting you to beat the game in one try or with saving/loading.
Everytime you die, you can respawn by simply pressing "use", wich will take away one of your lifes and puts you back to the starting point of the map.
You start out with 3 extralifes, but you can obtain more by buying them at the respective shop or sometimes even by finding some on the battlefield. Don't waste your lifes though, as buying new once is quite expensive and once you're out of extralifes and within a mission, you'll get sent to the jail!

In prison, there isn't really much you can do other than watch the monsters taking over the UAC outpost through some camera screens or retreat to the intermap through an open door, present in every map's jail. There you'll be able to buy new lifes, retry the mission or choosing a different one.

However, in multiplayer games, your comrades can get you out of the prison
by selecting a lifeitem through the inventory and using it.
This will activate a teleporter within the jail, allowing any player to return to the game!
You can use the lifeitem in singleplayer too, but it makes no sense whatsoever and definitely isn't recommended (unless you want to harm yourself)

3.3.The Time-attack mode
Once you have beaten a mission, you are given the chance to replay it in time-attack mode.
In this mode, you're trying to complete the mission as usual, just as fast as possible. If you happen to beat the map within the given time-limit, you're given an award in the form of extra credits.
After beating the time mode once, you still can replay the map as often as you want, but you won't get the bonus for completing it in time twice!
The time-attack mode has no influence on the progress of the game, so it is completely optional.

To replay a map either in time-attack mode or just for fun, just step through the entry door of the beaten map again.
Give it a try if you need some extra money or simply would like to play a map with a bit more of a challenge!

Completing this mission will give you no benefits (other than learning how things work)....

Does this mean that the money the player gets when entering the Mars station (INTERMAP) for the first time is given regardless of completing the training session? If not, then perhaps you should mention that the player gets this money for playing the training session.

Once you have beaten a mission, you are given the chance to replay it in time-attack mode.

Is a mission the same as a map? If so, the maps I've played have not given me the chance to replay them in time-attack mode. (Or at least I haven't gotten any message or other indication in this regard.)

Does this mean that the money the player gets when entering the Mars station (INTERMAP) for the first time is given regardless of completing the training session? If not, then perhaps you should mention that the player gets this money for playing the training session.

You mean the 300 credits? Yes, they are given from the start AFAIK (I can't check ATM)!

Is a mission the same as a map? If so, the maps I've played have not given me the chance to replay them in time-attack mode. (Or at least I haven't gotten any message or other indication in this regard.)

Yes, mission=map!
I didn't want to write "map" all over again and again in the article
You can replay a map in time-attack mode by just stepping through the respective door again!
I guess this is another of those things the player eventually will figure out, but I'll make an extra note about it in the initial post!

Vader wrote:You mean the 300 credits? Yes, they are given from the start AFAIK (I can't check ATM)!

I seem to recall that there were a huge number of credits upon completing the training map. In fact, in this post, Damage said:

"The godlike reward is given out to the player after completing the training mission. Now you probably know this already, but if you don't, well you should just disable all rewards for that mission. starting out with a cash prize of $7500 makes the first tier a walk in the park, not to mention that whenever I want more money I could just keep replaying the training mission over and over again."

I need to go back and check if the player can keep earning credits just by playing the training map over and over again. This would obviously not be desirable.